LC oscillator

ABSTRACT

An LC oscillator (I) comprises a cross-coupled PMOS transistor pair (Ma, Mb) coupled to a pair of capacitors (Cva, Cvb) and a pair of inductances (La, Lb). To enhance the signal amplification of the oscillator, a pair of auxiliary transistor circuits (Qa, Qb; Na, Nb) is provided which are coupled between the drain and, preferably, the source of each PMOS transistor. The capacitors (Cva, Cvb) are preferably variable capacitors and the inductances (La, Lb) are preferably connected to ground to allow a enlarged tuning voltage range.

The present invention relates to an LC oscillator. More in particular,the present invention relates to an LC oscillator for producing anoscillator signal, the oscillator comprising a resonance circuitcomprising a first and a second capacitor and a first and a secondinductance for determining the frequency of the oscillator signal, andan active circuit comprising a first and a second PMOS transistor foramplifying the oscillator signal, the gate of each said transistor beingdirectly coupled to the drain of the other transistor so as to provide adirectly cross-coupled transistor pair, wherein the active circuit iscoupled to the resonance circuit.

An oscillator of this kind defined above is disclosed in U.S. Pat. No.6,281,758. In the oscillator of U.S. Pat. No. 6,281,758 the resonancecircuit is labelled “varactor tuning circuit”, the capacitors beingconstituted by voltage controlled capacitors, also known as varactors.The inductors and capacitors are directly coupled to the cross-coupledPMOS transistors of the active circuit. The node connecting the twoinductors is connected to a buffering circuit containing an NMOStransistor which in turn is connected to ground.

In the design of LC oscillators MOS (Metal Oxide Semiconductor)transistors (also known as MOSFETs) have been preferred over bipolartransistors because of the MOS transistors' noise characteristics andsmooth transition from the saturated to the linear operating region, Theoscillator of U.S. Pat. No. 6,281,758 employs a pair of PMOS (p-channelMOS) transistors which introduce substantially lower flicker (1/f) noiseand a better substrate isolation compared to traditionally used NMOS(n-channel MOS) transistors. It has been found, however, that for agiven bias current the transconductance of the PMOS transistors is abouthalf that of their NMOS counterparts, resulting in a lower oscillatorsignal gain.

It is an object of the present invention to provide an LC oscillator asdefined in the preamble which has an improved oscillator signal gainwhile preserving the advantageous properties of the PMOS transistors.

Accordingly, an oscillator as defined above is in accordance with thepresent invention characterized in that the oscillator further comprisesa first and a second auxiliary transistor circuit for further amplifyingthe oscillator signal, each said auxiliary transistor circuit having aninput terminal coupled to the drain of the first and second PMOStransistor respectively.

That is, auxiliary transistor circuits are added to the LC oscillator soas to inprove its gain while leaving the basic configuration of theoscillator unaffected. In particular, the cross-coupled PMOS transistorarrangement is not altered. Instead, the auxiliary transistor circuitsare substantially external to the cross-coupled PMOS transistor pair,and coupled in such as way so as to not detrimentally influence thebasic noise properties of the oscillator.

In a preferred embodiment, the first and the second auxiliary transistorcircuit each have an output terminal coupled to the source of the firstand second PMOS transistor respectively. That is, the auxiliarytransistor circuits are each coupled between the drain and the source ofthe PMOS transistors. Preferably, the output terminals of the first andsecond auxiliary transistor circuit are coupled to the sources of thePMOS transistors through coupling capacitors.

In a particularly advantageous embodiment the first and second PMOStransistor are each provided with a back gate coupled with an outputterminal of a respective auxiliary transistor circuit. This provides anadditional signal path which assists in the amplification of theoscillator signal.

In an alternative embodiment, the first and second auxiliary transistorcircuit are each connected in series with the first and the second PMOStransistor respectively. That is, they are preferably directly coupledto the sources of the respective PMOS transistors.

Although it is preferred that the output terminals of the auxiliarytransistor circuits are coupled to the (sources of the) PMOStransistors, it can be envisaged that the first and the second auxiliarytransistor circuit each have an output terminal coupled to the resonancecircuit.

The first and the second auxiliary transistor circuit may each comprisea bipolar transistor, a MOS transistor, or both. Preferably, such a MOStransistor is an NMOS transistor.

Preferably, the first and/or the second capacitor is a variablecapacitor. This allows the oscillator frequency to be tuned. It is,however, also possible to use capacitors having a fixed value.

According to an important further aspect of the present invention it ispreferred that the resonance circuit is directly coupled to ground. Thisis particularly advantageous in embodiments in which the resonancecircuit contains variable capacitors. Such capacitors require a tuningvoltage which can be varied over a much wider range when the resonancecircuit is coupled to ground than otherwise would be the case. Thus awider frequency tuning range is obtained. In addition, the (anode)terminals of the variable capacitors are thus provided with a direct DCcoupling to ground, resulting in a tuning voltage having a very lownoise and hence an oscillator signal having a high spectral purity.

Coupling the resonance circuit directly to ground may also be effectedin non-tuneable LC oscillators, that is, oscillators having fixedcapacitor values.

The present invention further provides an integrated circuit comprisingan LC oscillator as defined above, as well as a device comprising an LCoscillator as defined above. The oscillator of the present invention isparticularly suitable for applications where a relatively low supplyvoltage is used. Exemplary devices in which the oscillator of thepresent invention can be advantageously used are, but are not limitedto, television tuners, channel converters and similar telecommunicationsdevices.

The present invention will further be explained below with reference toexemplary embodiments illustrated in the accompanying drawings, inwhich:

FIG. 1 schematically shows a first embodiment of an oscillator accordingto the present invention.

FIG. 2 schematically shows a second embodiment of an oscillatoraccording to the present invention.

FIG. 3 schematically shows a third embodiment of an oscillator accordingto the present invention.

FIG. 4 schematically shows a fourth embodiment of an oscillatoraccording to the present invention.

The oscillator 1 shown merely by way of non-limiting example in FIG. 1comprises a resonance circuit, an active circuit, and two auxiliarytransistor circuits. The resonance circuit (which may also be referredto as tank circuit) comprises a first inductor La and a second inductorLb which are connected in series, as well as a first capacitor Cva and asecond capacitor Cvb which are also connected in series, parallel to theinductors so as to form a closed loop. The capacitors Cva and Cvb arevariable capacitors and receive a tuning voltage Vtun at their commonnode. The common node of the inductors is shown to be connected toground, as will later be explained in more detail below.

The active circuit comprises two PMOS transistors Ma and Mb which arecross-coupled, that is, the gate of transistor Mb is connected to thedrain of transistor Ma, while the gate of transistor Ma is connected tothe drain of transistor Mb. The drains of the first and second PMOStransistors Ma and Mb are also coupled to the first and secondcapacitors Cva, Cvb and first and second inductors La and Lb,respectively. The sources of the PMOS transistors Ma and Mb areconnected to current sources providing (equal) bias currents.

In accordance with the present invention, the oscillator circuit 1further comprises two auxiliary transistor circuits (or auxiliaryamplification circuits) comprising transistors Qa and Qb respectively.These transistors Qa and Qb, which are shown to be bipolar npn-typetransistors, each have a base which is coupled, via a coupling capacitorCBa, CBb, to the drain of the respective PMOS transistor. The nodeconnecting the coupling capacitor CBa, CBb to the PMOS transistor Ma, Mbconstitutes the input terminal of the respective auxiliary transistorcircuit, while the output terminal is constituted by the node connectinga further coupling capacitor CEa, CEb to the source of the PMOStransistor Ma, Mb. This further coupling capacitor CEa, CEb is connectedto the emitter of the transistor Qa, Qb, thus forming an emitterfollower circuit.

Also connected to the emitter of the transistors Qa, Qb is a back gateof the PMOS transistors Ma, Mb. The back gate or “bulk connection” is aparasitic MOS transistor terminal which provides a further opportunityfor signal control.

As can be seen from FIG. 1, the basic configuration of the LC oscillator1 of FIG. 1 is identical to the one known from the Prior Art. However,the auxiliary transistor circuits comprising transistors Qa and Qbprovide additional gain. This additional gain is achieved withoutaffecting the excellent noise characteristics of the basic oscillatorcircuit.

As stated above, the resonance circuit (Cva, Cvb, La, Lb) of theembodiment of FIG. 1 is directly connected to ground. This allows thetuning voltage Vtun fed to the variable capacitors (varactors) Cva, Cvbto range from the supply voltage Vcc to a minimum voltage of about 0.3 Vnecessary to bias the varactors. This tuning range is substantiallylarger than in Prior Art LC oscillators where the resonance circuit istypically at a voltage ranging between the supply voltage and ground.Means to artificially connect anode terminals of the variable capacitorsto ground using AC coupling have a detrimental effect on spectral purityand on the frequency tuning range. As a result, the oscillator of thepresent invention has a substantially larger tuning range.

In the example shown the auxiliary transistor circuits each comprise asingle bipolar transistor. It is, however, possible to use a MOStransistor instead of the bipolar transistor, or to use multipletransistors in each auxiliary transistor circuit.

Although the preferred embodiment of the oscillator 1 of the presentinvention is shown in FIG. 1, alternative embodiments are possible. Onesuch alternative embodiment is shown in FIG. 2. This embodiment issimilar to the one shown in FIG. 1. However, the output terminals of theauxiliary transistor circuits are not connected to the sources of thePMOS transistors but are instead connected to ground. The sources of thePMOS transistors are mutually connected since no feedback loop betweenthe sources and the drains is available via the auxiliary transistorcircuits. The two bipolar transistors independently add to the overallamplification, contributing in the same manner as the well knownCollpits oscillator. Similar to the embodiment of FIG. 1, the back gatesof the PMOS transistors can advantageously be connected to therespective emitters of the bipolar transistors (not shown).

In the embodiment of FIG. 3 the auxiliary transistors Qa, Qb arearranged in series with the PMOS transistors Ma, Mb: the emitters of Qa,Qb are connected to the sources of Ma, Mb. This embodiment has theadvantage of requiring fewer components and, by sharing bias currents(I_(DC)), being more power efficient.

The embodiment of FIG. 4 is substantially identical to that of FIG. 3,with the exception of the bipolar transistors Qa, Qb which have beenreplaced with NMOS transistors Na, Nb.

As will be clear from the above description, the present invention isbased upon the insight that the advantageous noise properties of PMOStransistors should be preserved while providing an LC oscillator havinga higher gain. This higher gain means that the applications of theoscillator of the present invention are not restricted to resonancecircuits having a high quality factor (high resonance impedance) andthat resonance circuits having a lower quality factor may also be used.In addition, a greater safety margin is provided against no oscillationoccurring in extreme operating conditions which may result fromvariations in the supply voltage, variations in temperature andparameter changes which in turn may result from variations in themanufacturing process of the integrated cicuits in which the oscillatorsmay be embodied. The LC oscillator of the present invention keeps thebasic oscillator configuration intact but adds transistors to increasethe signal gain.

It is noted that in this document the word “comprising” is not meant toexclude other elements or steps, and that the use of the word “an” or“a” does not exclude a plurality. A single transistor or other circuitelement may fulfill the function or functions of two of more circuitelements or means recited in the claims. In particular, the twoinductances recited in the claims may be replaced by a suitable singleinductance. Conversely, the single transistors Qa, Qb, Na and/or Nbshown in the drawings may each be replaced by a transistor circuitconsisting of two or more transistors.

It will therefore be understood by those skilled in the art that thepresent invention is not limited to the embodiments illustrated aboveand that many modifications and additions may be made without departingfrom the scope of the invention as defined in the appending claims.

1. An LC oscillator for producing an oscillator signal, the oscillatorcomprising: a resonance circuit comprising a first and a secondcapacitor and a first and a second inductance for determining thefrequency of the oscillator signal, and an active circuit comprising afirst and a second PMOS transistor for amplifying the oscillator signal,the gate of each said transistor being directly coupled to the drain ofthe other transistor so as to provide a directly cross-coupledtransistor pair, wherein the active circuit is coupled to the resonancecircuit, characterized in that the oscillator further comprises a firstand a second auxiliary transistor circuit for further amplifying theoscillator signal, each said auxiliary transistor circuit having aninput terminal capacitively coupled to the drain of the first and secondPMOS transistor respectively.
 2. An LC oscillator according to claim 1,wherein the first and the second auxiliary transistor circuits each havean output terminal coupled to the source of the first and second PMOStransistor respectively.
 3. An LC oscillator according to claim 2,wherein the output terminals of the first and second auxiliarytransistor circuit are coupled to the sources of the PMOS transistorsthrough coupling capacitors.
 4. An LC oscillator according to claim 3,wherein the first and second PMOS transistor are each provided with acontrollable back gate coupled with an output terminal of a respectiveauxiliary transistor circuit.
 5. An LC oscillator according to claim 3,wherein the first and second auxiliary transistor circuit are connectedin series with the first and the second PMOS transistor respectively. 6.An LC oscillator according to claim 1, wherein the first and the secondauxiliary transistor circuits each have an output terminal coupled tothe resonance circuit.
 7. An LC oscillator according to claim 1, whereinthe first and the second auxiliary transistor circuit comprise a bipolartransistor.
 8. An LC oscillator according to claim 1, wherein the firstand the second auxiliary transistor circuit comprise a MOS transistor,preferably an NMOS transistor.
 9. An LC oscillator according to claim 1,wherein the first and/or the second capacitor is a variable capacitor.10. An LC oscillator according to claim 1, wherein the resonance circuitis directly coupled to ground.
 11. An integrated circuit, comprising anLC oscillator according to claim
 1. 12. A device comprising an LCoscillator for producing an oscillator signal, the oscillatorcomprising: a resonance circuit comprising a first and a secondcapacitor and a first and a second inductance for determining thefrequency of the oscillator signal, and an active circuit comprising afirst and a second PMOS transistor for amplifying the oscillator signal,the gate of each said transistor being directly coupled to the drain ofthe other transistor so as to provide a directly cross-coupledtransistor pair, wherein the active circuit is coupled to the resonancecircuit, characterized in that the oscillator further comprises a firstand a second auxiliary transistor circuit for further amplifying theoscillator signal, each said auxiliary transistor circuit having aninput terminal capacitively coupled to the drain of the first and secondPMOS transistor respectively.
 13. A device according to claim 12 whichis a television or telecommunications device.